A TFT (Thin Film Transistor) has been widely used for liquid crystal display devices, organic EL (Electro Luminescence) display devices, etc.
Although an amorphous silicon TFT used for large liquid crystal displays has a problem in reliability of prolonged drive because a field effect mobility (mobility) thereof is approximately 1 cm2/V·s, the TFT can be formed over a large area, at low cost, and uniformly because it can be formed with plasma CVD (Chemical Vapor Deposition) suitable for film formation over the large area.
In addition, although a low temperature polysilicon TFT used for small and medium-sized liquid crystal displays has a problem in cost reduction and uniformity, the TFT has high reliability of prolonged drive because a mobility thereof is approximately 100 cm2/V·s.
In recent years, further increase in size, high definition, and cost reduction of display devices have been desired. In addition to that, an active-matrix type organic EL display device utilizing current drive has been studied and developed at home and abroad, and a new semiconductor material has been desired which can be formed at low cost and which has high uniformity, high reliability, and a high mobility.
Consequently, in recent years, an oxide semiconductor has been attracting attention as a semiconductor film material of a TFT. In particular, a TFT using an amorphous oxide semiconductor, such as IGZO (In—Ga—Zn—O) has been attracting attention. Since a film of the oxide semiconductor can be uniformly formed over a large area at room temperature by a sputtering method and is transparent in a visible light region, it is possible to form a flexible and transparent TFT even on a plastic film substrate with low heat resistance.
Further, since the oxide semiconductor has a mobility approximately ten times higher than amorphous silicon, a high mobility characteristic can be obtained by using the oxide semiconductor. In addition, high reliability can be obtained with respect to a BTS (Bias Temperature Stress) test by applying post-annealing to the oxide semiconductor with a high temperature of 300 to 400° C. Such features have made the oxide semiconductor a front-runner as a semiconductor material used for a next-generation back plane device of a display device.